In the opening scene of "Tammy," Melissa McCarthy attempts to perform mouth-to-mouth resuscitation on a deer she's just hit with her car. It's a surreal and endearing intro, but it turns out to be the best moment in this grating attempt at comedy, the latest failed attempt to capitalize on McCarthy's considerable charm.

Hitting that deer ends up costing Tammy her fast-food job and her beat-up Toyota Corolla, and when she returns home to find her husband (Nat Faxon) enjoying a pleasant home-cooked meal with another woman (Toni Collette), she decides to get the heck out of the small Illinois town where she lives down the street from her mother (Allison Janney). Tammy's irascible, alcoholic grandmother Pearl (Susan Sarandon) has a wad of cash and a working auto, so the pair head off on a road trip.

The main problem (but not by far the only one) with "Tammy" is that these are two of the most annoying, pitiable characters you could imagine being trapped in a car with. Tammy is lazy, rude, juvenile, and a slob, much like McCarthy's characters in "Identity Thief" and "The Heat." The grey-wigged, loose-limbed Sarandon calls to mind a boozy version of Vicki Lawrence in the old TV series "Mama's Family," and the movie's slapstick, sizeist humor is about as sophisticated as that moronic show.

After drunken shenanigans on a dairy farm and a jet-ski mishap, the duo's efforts to reach Niagara Falls get sidetracked in Louisville, Kentucky, where they meet a grey-bearded Lothario (Gary Cole) and his mild-mannered son (Mark Duplass, looking embarrassed to be here). Later they end up at a "lesbian Fourth of July party" (is that actually a thing?) hosted by Pearl's cousin (Kathy Bates) and her partner (Sandra Oh).

Using an earthy, uncouth character to puncture pretension is a time-honored comedy tradition, and sometimes it feels like "Tammy" wants to be a version of "Roseanne." But Roseanne never inspired pity the way Tammy does when, for instance, she's forced by her horny grandmother to sleep outside their motel room, gnawing on a powdered donut next to a raccoon. That's not the only scene that's played for laughs but just feels sad.

Instead of an empowering working-class heroine, Tammy comes off as a cynical, condescending creation, which is especially disappointing considering that McCarthy and her husband, Ben Falcone, wrote the film, with Falcone directing (if that's the word for it). McCarthy's star rose following her raunchy performance in "Bridesmaids," but now she feels stuck in a lowbrow rut of pratfalls and romantic humiliation.

Near the end of this overlong 96-minute movie, Bates (in her now-typical lesbian earth mother role) tells Tammy "I got a news flash for you: Life isn't fair." (Yes, that's actually the moral of this story.) Eventually, McCarthy's character starts to turn her life around — here's hoping she can do the same for her career after this stinker.

The lowdown: After losing her job, her car, and her husband, an obnoxious woman (Melissa McCarthy) heads out on a heartland road trip with her alcoholic grandmother (Susan Sarandon). The laughs are few and far between in this cynical, condescending comedy.